This invention relates to catalyst recycle, and more particularly to the conversion and recycle of the constituents of catalyst compositions used to conduct chemical reactions.
The use as catalysts of various transition metals is known. Among the catalyst constituents in common use are compounds of the Group VIIIB metals, including those having atomic numbers of at least 44 (i.e., ruthenium, rhodium, palladium, osmium, iridium and platinum), hereinafter "heavy Group VIIIB metals".
Compounds of other transition metals such as cobalt, lead, manganese, cerium and copper are also useful as constituents of catalyst compositions. Those of the Group VIIIB metals having atomic numbers less than 44 (i.e., iron, cobalt and nickel), hereinafter "light Group VIIIB metals", are often especially useful and valuable.
Bromide sources are also frequently present in various catalyst compositions. These include trialkylamine hydrobromides and tetraalkylammonium, tetraalkylphosphonium and hexaalkylguanidinium bromides.
The reactions in which such compounds serve a catalytic function include some which involve organic compounds as reactants, products or both. An illustration is the catalytic carbonylation of hydroxyaromatic compounds such as phenol with carbon monoxide and oxygen to yield diaryl carbonates such as diphenyl carbonate. This reaction will sometimes be designated "carbonylation" hereinafter.
In a typical carbonylation reaction, phenol is combined with a compound of a heavy Group VIIIB metal, most often palladium, and other catalytic species which may include organic and inorganic co-catalysts and at least one bromide source. Light Group VIIIB metal, especially cobalt, compounds are often used as inorganic cocatalysts, and the use of tetraalkylammonium or hexaalkylguanidinium bromides as bromide sources is frequently advantageous. The resulting mixture is pressurized with carbon monoxide and oxygen to yield a product mixture containing diphenyl carbonate, unreacted phenol and by-products which include palladium in elemental and/or combined form and cobalt compounds.
Since transition metals such as palladium and cobalt and bromide sources are expensive, it would be highly desirable to recover them from the product mixture, after which they could be reconverted to catalytically active species capable of employment in further carbonylations.